This invention relates to submersible vessels such as immersed submarines and more particularly to an automatic specific gravity compensation system for a submersible vessel or object.
The invention may be used with any vessel or object which is immersed in a fluid medium, but will be described in particular for use with a submersible vessel such as ocean research vessels which have some motive power and which may operate at very great depths in water.
It is well known that once submerged, the specific gravity of a vessel changes due to density changes of the surrounding ocean or to volume changes caused by either elastic strain or thermal deformation of the pressure hull or of any auxiliary flotation materials.
Water density, the change of which is mostly due to compressibility, is a function of operating depth. Volume changes of pressure hulls in general vary linearly with depth as most engineering materials are used in the region below their proportional limit. Most flotation materials, however, are similar to water and have a Bulk Modulus which increases somewhat with pressure allowing for smaller volume changes for the same increment of pressure at the higher pressures.
For typical design stresses, the metallic pressure hull of a submersible is from two to three times as stiff as the surrounding water. Auxiliary flotation materials such as syntactic foam have the same order of stiffness. Other materials such as solid glass can be highly stressed so that their volume changes may be made somewhat equal to or greater than water. Most liquid flotation materials are more compressible than water.
An ideally designed submersible has a specific gravity which will change proportionately the same as the surrounding ocean water when diving. This may be accomplished with a judicious selection of materials but a the expense of having a vehicle which may either be inefficiently designed or functionally inferior. Up until the present, a typical deep diving submersible contained either a pressure hull alone for flotation, or a combination of pressure hull and some other solid or liquid material for flotation. If the pressure hull or the combination pressure hull and solid material are used for flotation, some ballasting system has to be used to remove the additional buoyancy the submersible gets on going to the ocean bottom. In general, the system uses flooding tanks or tanks where a liquid lighter than water is released and replaced by sea water. After completing an operation in the deep ocean some weight is released to get back the buoyancy required to ascend to the surface. As an example, for the submersible ALUMINAUT to ascent from a working depth of 15,000 ft. approximately 4500 lbs of steel shot are dropped. For all vehicles, if the depth variation is required while working near the ocean bottom additional ballasting is required.
If a liquid flotation system is used, the process is reversed. Shot is dropped to remain slightly negatively buoyant while descending to a working depth. To surface, a slight additional amount of shot is released to obtain a slight positive buoyance. The submersible then gains additional buoyancy upon rising due to the greater than sea-water expansion of the flotation fluid. Again, if depth variation is wanted additional shot is required. In general, much more shot is required for a vehicle of this type due to its larger displacement to support a given payload.
A number of different schemes for varying specific gravity have been considered. For shallow and medium depths stored air systems have proven advantageous. For higher pressures, some sort of pumping system is required. To pump against a high pressure head, takes considerable energy. It is estimated that at a 20,000 ft. depth it would take a pound of batteries to pump 3 or 4 lbs of water, to say nothing of the equipment required. One such system is set forth in U.S. Pat. No. 3,665,884 which makes use of a hydraulic pump, storage accummulator system which controls water ballast storage.